He studied mathematics and physics at University of California, Los Angeles before joining the United States Army after the Japanese attack on Pearl Harbor.

[3] In 1944 he worked on the Manhattan Project in the efficiency group at Los Alamos and calculated how neutrons behaved in Fat Man, the atomic bomb that was later detonated over Nagasaki, Japan.

[3] At RAND Corporation in 1950, his work on the intensity of fallout radiation first became public when his calculations were included as a special appendix in Samuel Glasstone's book The Effects of Atomic Weapons.

The use of clean weapons in strategic situations may be indicated in order to protect the local population.– Dr. Hans Bethe, Working Group Chairman, 27 March 1958 "Top Secret – Restricted Data" Report to the NSC Ad Hoc Working Group on the Technical Feasibility of a Cessation of Nuclear Testing, p 9.In consequence of Bethe's recommendations, on July 12, 1958, the Hardtack-Poplar shot of the Mk-41C warhead was carried out on a barge in the lagoon yielded 9.3 megatons, of which only 4.8% was fission, and thus 95.2% "clean".

So a larger percentage of neutrons escapes from a small detonation, due to the thinner case required to reflect back X-rays during the secondary stage (fusion) ignition.

In 1992 he was featured in the award-winning BBC TV series Pandora's Box, episode "To the Brink of Eternity", discussing his battles with officialdom and colleagues at the RAND Corporation.

Cohen controversially argued: "When we started this systems analysis business, we stepped through the looking glass where people did the weirdest things and (used) the most perverse kind of logic imaginable and yet claimed to have the most precise understanding of everything.

[citation needed] The speed of modern warfare meant that the civilian population would be unlikely to be able to withdraw from combat zones and would suffer a large number of deaths in a nuclear war where the blast yields and fallout were significant.

If the "conventional story" is to be believed, red mercury was a disinformation campaign led by US government agencies in order to lure potential terrorists into being captured.

A reiteration of the claim can be seen in The Nuclear Threat That Doesn't Exist – or Does It or No?, by Cohen and Joe Douglass in a March 11, 2003, guest editorial in Financial Sense Online.

[11] In 1984, in a cover story for the libertarian magazine Reason, Cohen expressed support for the construction of a nuclear-radiation field along the borders of Israel as a means of protecting it from the military forces of other nations and killing anyone who approached it via gamma radiation.

The JDL posted Cohen's home phone number and address on its website, urging its members to contact him, to persuade him to stop supporting Buchanan.

[18] As part of a self-described unusual friendship, Cohen wrote the afterword to William P. Grady's 2005 bestseller titled How Satan Turned America Against God.

Cohen explained that although he was an unbelieving Jew, and thus could not relate to the spiritual content of the book, he concurred with Grady's grasp of America's disastrous foreign policy.